Systems and methods for performing surgical procedures and assessments

ABSTRACT

The present invention involves systems and related methods for performing surgical procedures and assessments, including the use of neurophysiology-based monitoring to: (a) determine nerve proximity and nerve direction to surgical instruments employed in accessing a surgical target site; (b) assess the pathology (health or status) of a nerve or nerve root before, during, or after a surgical procedure; and/or (c) assess pedicle integrity before, during or after pedicle screw placement, all in an automated, easy to use, and easy to interpret fashion so as to provide a surgeon-driven system.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.12/628,549 filed by Gharib et al. on Dec. 1, 2009, now issued as U.S.Pat. No. 8,000,782 (the contents being incorporated herein byreference), which is a division of U.S. patent application Ser. No.12/426,792 filed by Gharib et al. on Apr. 20, 2009 now U.S. Pat. No.8,027,716 (the contents being incorporated herein by reference), whichis a continuation of U.S. patent application Ser. No. 10/809,280 filedby Gharib et al. on Mar. 25, 2004, now issued as U.S. Pat. No. 7,522,953(the contents being incorporated herein by reference), which is acontinuation of PCT Patent Application Ser. No. PCT/US02/30617 filed onSep. 25, 2002 and published as WO 03/026482 (the contents beingincorporated herein by reference).

BACKGROUND

I. Field of the Invention

The present invention relates to a system and methods generally aimed atsurgery. More particularly, the present invention is directed at asystem and related methods for performing surgical procedures andassessments involving the use of neurophysiology.

II. Description of Related Art

A variety of surgeries involve establishing a working channel to gainaccess to a surgical target site. Oftentimes, based on the anatomicallocation of the surgical target site (as well as the approach thereto),the instruments required to form or create or maintain the workingchannel may have to pass near or close to nerve structures which, ifcontacted or disturbed, may be problematic to the patient. Examples ofsuch “nerve sensitive” procedures may include, but are not necessarilylimited to, spine surgery and prostrate or urology-related surgery.

Systems and methods exist for monitoring nerves and nerve muscles. Onesuch system determines when a needle is approaching a nerve. The systemapplies a current to the needle to evoke a muscular response. Themuscular response is visually monitored, typically as a shake or“twitch.” When such a muscular response is observed by the user, theneedle is considered to be near the nerve coupled to the responsivemuscle. These systems require the user to observe the muscular response(to determine that the needle has approached the nerve). This may bedifficult depending on the competing tasks of the user. In addition,when general anesthesia is used during a procedure, muscular responsemay be suppressed, limiting the ability of a user to detect theresponse.

While generally effective (although crude) in determining nerveproximity, such existing systems are incapable of determining thedirection of the nerve to the needle or instrument passing throughtissue or passing by the nerves. This can be disadvantageous in that,while the surgeon may appreciate that a nerve is in the generalproximity of the instrument, the inability to determine the direction ofthe nerve relative to the instrument can lead to guess work by thesurgeon in advancing the instrument and thereby raise the specter ofinadvertent contact with, and possible damage to, the nerve.

Another nerve-related issue in existing surgical applications involvesthe use of nerve retractors. A typical nerve retractor serves to pull orotherwise maintain the nerve outside the area of surgery, therebyprotecting the nerve from inadvertent damage or contact by the “active”instrumentation used to perform the actual surgery. While generallyadvantageous in protecting the nerve, it has been observed that suchretraction can cause nerve function to become impaired or otherwisepathologic over time due to the retraction. In certain surgicalapplications, such as spinal surgery, it is not possible to determine ifsuch retraction is hurting or damaging the retracted nerve until afterthe surgery (generally referred to as a change in “nerve health” or“nerve status”). There are also no known techniques or systems forassessing whether a given procedure is having a beneficial effect on anerve or nerve root known to be pathologic (that is, impaired orotherwise unhealthy).

In spinal surgery, and specifically in spinal fusion procedures, a stillfurther nerve-related issue exists with regard to assessing theplacement of pedicle screws. More specifically, it has been founddesirable to detect whether the medial wall of a pedicle has beenbreached (due to the formation of the hole designed to receive a pediclescrew or due to the placement of the pedicle screw into the hole) whileattempting to effect posterior fixation for spinal fusion through theuse of pedicle screws. Various attempts have been undertaken atassessing the placement of pedicle screws. X-ray and other imagingsystems have been employed, but these are typically quite expensive andare oftentimes limited in terms of resolution (such that pediclebreaches may fail to be detected).

Still other attempts involve capitalizing on the insulatingcharacteristics of bone (specifically, that of the medial wall of thepedicle) and the conductivity of the exiting nerve roots themselves.That is, if the medial wall of the pedicle is breached, a stimulationsignal (voltage or current) applied to the pedicle screw and/or thepre-formed hole (prior to screw introduction) will cause the variousmuscle groups coupled to the exiting nerve roots to twitch. If thepedicle wall has not been breached, the insulating nature of the medialwall will prevent the stimulation signal from innervating the givennerve roots such that the muscle groups will not twitch.

To overcome this obviously crude technique (relying on visible musclestwitches), it has been proposed to employ electromyographic (EMG)monitoring to assess whether the muscle groups in the leg areinnervating in response to the application of a stimulation signal tothe pedicle screw and/or the pre-formed hole. This is advantageous inthat it detects such evoked muscle action potentials (EMAPs) in the legmuscles as much lower levels than that via the “visual inspection”technique described above. However, the traditional EMG systems employedto date suffer from various drawbacks. First, traditional EMG systemsused for pedicle screw testing are typically quite expensive. Moreimportantly, they produce multiple waveforms that must be interpreted bya neurophysiologist. Even though performed by specialists, interpretingsuch multiple EMG waveforms in this fashion is nonethelessdisadvantageously prone to human error and can be disadvantageously timeconsuming, adding to the duration of the operation and translating intoincreased health care costs. Even more costly is the fact that theneurophysiologist is required in addition to the actual surgeonperforming the spinal operation.

The present invention is directed at eliminating, or at least reducingthe effects of, the above-described problems with the prior art.

SUMMARY

The present invention includes a system and related methods forperforming surgical procedures and assessments, including the use ofneurophysiology-based monitoring to: (a) determine nerve proximity andnerve direction to surgical instruments employed in accessing a surgicaltarget site; (b) assess the pathology (health or status) of a nerve ornerve root before, during, or after a surgical procedure; and/or (c)assess pedicle integrity before, during or after pedicle screwplacement, all in an automated, easy to use, and easy to interpretfashion so as to provide a surgeon-driven system.

The present invention accomplishes this by combining neurophysiologymonitoring with any of a variety of instruments used in or inpreparation for surgery (referred to herein as “surgical accessories”).By way of example only, such surgical accessories may include, but arenot necessarily limited to, any number of devices or components forcreating an operative corridor to a surgical target site (such asK-wires, sequentially dilating cannula systems, distractor systems,and/or retractor systems), devices or components for assessing pedicleintegrity (such as a pedicle testing probe), and/or devices orcomponents for retracting or otherwise protecting a nerve root before,during and/or after surgery (such as a nerve root retractor). Althoughdescribed herein largely in terms of use in spinal surgery, it is to bereadily appreciated that the teachings of the method and apparatus ofthe present invention are suitable for use in any number of additionalsurgical procedures wherein tissue having significant neural structuresmust be passed through (or near) in order to establish an operativecorridor to a surgical target site, wherein neural structures arelocated adjacent bony structures, and/or wherein neural structures areretracted or otherwise contacted during surgery.

The fundamental method steps according to the present invention include:(a) stimulating one or more electrodes provided on a surgical accessory;(b) measuring the response of nerves innervated by the stimulation ofstep (a); (c) determining a relationship between the surgical accessoryand the nerve based upon the response measured in step (b); andcommunicating this relationship to the surgeon in an easy-to-interpretfashion.

The step of stimulating may be accomplished by applying any of a varietyof suitable stimulation signals to the electrode(s) on the surgicalaccessory, including voltage and/or current pulses of varying magnitudeand/or frequency. The stimulating step may be performed at differenttimes depending upon the particular surgical accessory in question. Forexample, when employed with a surgical access system, stimulation may beperformed during and/or after the process of creating an operativecorridor to the surgical target site. When used for pedicle integrityassessments, stimulation may be performed before, during and/or afterthe formation of the hole established to receive a pedicle screw, aswell as before, during and/or after the pedicle screw is introduced intothe hole. With regard to neural pathology monitoring, stimulation may beperformed before, during and/or after retraction of the nerve root.

The step of measuring the response of nerves innervated by thestimulation step may be performed in any number of suitable fashions,including but not limited to the use of evoked muscle action potential(EMAP) monitoring techniques (that is, measuring the EMG responses ofmuscle groups associated with a particular nerve). According to oneaspect of the present invention, the measuring step is preferablyaccomplished via monitoring or measuring the EMG responses of themuscles innervated by the nerve(s) stimulated in step for each of thepreferred functions of the present invention: surgical access, pedicleintegrity assessments, and neural pathology monitoring.

The step of determining a relationship between the surgical accessoryand the nerve based upon the measurement step may be performed in anynumber of suitable fashions depending upon the manner of measuring theresponse, and may define the relationship in any of a variety offashions (based on any number of suitable parameters and/orcharacteristics). By way of example only, the step of determining arelationship, within the context of a surgical access system, mayinvolve identifying when (and preferably the degree to which) thesurgical accessory comes into close proximity with a given nerve (“nerveproximity”) and/or identifying the relative direction between thesurgical accessory and the nerve (“nerve direction”). For a pedicleintegrity assessment, the relationship between the surgical accessory(screw test probe) and the nerve is whether electrical communication isestablished therebetween. If electrical communication is established,this indicates that the medial wall of the pedicle has been cracked,stressed, or otherwise breached during the steps of hole formationand/or screw introduction. If not, this indicates that the integrity ofthe medial wall of the pedicle has remained intact during hole formationand/or screw introduction. This characteristic is based on theinsulating properties of bone. For neural pathology assessmentsaccording to the present invention, the relationship may be, by way ofexample only, whether the neurophysiologic response of the nerve haschanged over time. Such neurophysiologic responses may include, but arenot necessarily limited to, the onset stimulation threshold for thenerve in question, the slope of the response vs. the stimulation signalfor the nerve in question and/or the saturation level of the nerve inquestion. Changes in these parameters will indicate if the health orstatus of the nerve is improving or deteriorating, such as may resultduring surgery.

The step of communicating this relationship to the surgeon in aneasy-to-interpret fashion may be accomplished in any number of suitablefashions, including but not limited to the use of visual indicia (suchas alpha-numeric characters, light-emitting elements, and/or graphics)and audio communications (such as a speaker element). By way of exampleonly, with regard to surgical access systems, this step of communicatingthe relationship may include, but is not necessarily limited to,visually representing the stimulation threshold of the nerve (indicatingrelative distance or proximity to the nerve), providing color codedgraphics to indicate general proximity ranges (i.e. “green” for a rangeof stimulation thresholds above a predetermined safe value, “red” forrange of stimulation thresholds below a predetermined unsafe value, and“yellow” for the range of stimulation thresholds in between thepredetermined safe and unsafe values—designating caution), as well asproviding an arrow or other suitable symbol for designating the relativedirection to the nerve. This is an important feature of the presentinvention in that, by providing such proximity and directioninformation, a user will be kept informed as to whether a nerve is tooclose to a given surgical accessory element during and/or after theoperative corridor is established to the surgical target site. This isparticularly advantageous during the process of accessing the surgicaltarget site in that it allows the user to actively avoid nerves andredirect the surgical access components to successfully create theoperative corridor without impinging or otherwise compromising thenerves. Based on these nerve proximity and direction features, then, thepresent invention is capable of passing through virtually any tissuewith minimal (if any) risk of impinging or otherwise damaging associatedneural structures within the tissue, thereby making the presentinvention suitable for a wide variety of surgical applications.

With regard to pedicle integrity assessments, the step of communicatingthe relationship may include, but is not necessarily limited to,visually representing the actual stimulation threshold of an exitingnerve root alone or in combination with the stimulation threshold of abare nerve root (with or without the difference therebetween), as wellas with providing color coded graphics to indicate general ranges ofpedicle integrity (i.e. “green” for a range of stimulation thresholdsabove a predetermined safe value—indicating “breach unlikely”, “red” forrange of stimulation thresholds below a predetermined unsafevalue—indicating “breach likely”, and “yellow” for the range ofstimulation thresholds between the predetermined safe and unsafevalues—indicating “possible breach”). This is a significant feature, andadvantage over the prior art, in that it provides a straightforward andeasy to interpret representation as to whether a pedicle has beenbreached during and/or after the process of forming the hole and/orintroducing the pedicle screw. Identifying such a potential breach ishelpful in that it prevents or minimizes the chance that a misplacedpedicle screw (that is, one breaching the medial wall) will be misseduntil after the surgery. Instead, any such misplaced pedicle screws,when stimulated according to the present invention, will produce an EMGresponse at a myotome level associated with the nerve in close proximityto the pedicle screw that is breaching the pedicle wall. This willindicate to the surgeon that the pedicle screw needs to be repositioned.But for this system and technique, patients may be released andsubsequently experience pain due to the contact between the exitingnerve root and the pedicle screw, which oftentimes requires anothercostly and painful surgery.

As for neural pathology monitoring, the step of communicating therelationship may include, but is not necessarily limited to, visuallyrepresenting the changes over time in the onset stimulation threshold ofthe nerve, the slope of the response versus the stimulation threshold ofthe nerve and/or the saturation level of the nerve. Once again, thesechanges may indicate if the health or status of the nerve is improvingor deteriorating, such as may result during surgery and/or retraction.This feature is important in that it may provide qualitative feedback onthe effect of the particular surgery. If it appears the health or status(pathology) of the nerve is deteriorating over time, the user may beinstructed to stop or lessen the degree of retraction to avoid suchdeterioration. If the pathology of the nerve improves over time, it mayindicate the success of the surgery in restoring or improving nervefunction, such as may be the case in decompressive spinal surgery.

The present invention also encompasses a variety of techniques,algorithms, and systems for accomplishing the steps of (a) stimulatingone or more electrodes provided on a surgical accessory; (b) measuringthe response of nerves innervated by the stimulation of step (a); (c)determining a relationship between the surgical accessory and the nervebased upon the response measured in step (b); and/or communicating thisrelationship to the surgeon in an easy-to-interpret fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating the fundamental steps of theneurophysiology-based surgical system according to the presentinvention;

FIG. 2 is a perspective view of an exemplary surgical system 20 capableof determining nerve proximity and direction to surgical instrumentsemployed in accessing a surgical target site, assessing pedicleintegrity before, during or after pedicle screw placement, and/orassessing the pathology (health and/or status) of a nerve or nerve rootbefore, during, or after a surgical procedure;

FIG. 3 is a block diagram of the surgical system 20 shown in FIG. 2;

FIG. 4 is a graph illustrating a plot of a stimulation current pulsecapable of producing a neuromuscular response (EMG) of the type shown inFIG. 3;

FIG. 5 is a graph illustrating a plot of the neuromuscular response(EMG) of a given myotome over time based on a current stimulation pulse(such as shown in FIG. 4) applied to a nerve bundle coupled to the givenmyotome;

FIG. 6 is an illustrating (graphical and schematic) of a method ofautomatically determining the maximum frequency (F_(Max)) of thestimulation current pulses according to one embodiment of the presentinvention;

FIG. 7 is a graph illustrating a plot of EMG response peak-to-peakvoltage (V_(pp)) for each given stimulation current level (I_(Stim))forming a stimulation current pulse according to the present invention(otherwise known as a “recruitment curve”);

FIG. 8 is a graph illustrating a traditional stimulation artifactrejection technique as may be employed in obtaining each peak-to-peakvoltage (V_(pp)) EMG response according to the present invention;

FIG. 9 is a graph illustrating the traditional stimulation artifactrejection technique of FIG. 8, wherein a large artifact rejection causesthe EMG response to become compromised;

FIG. 10 is a graph illustrating an improved stimulation artifactrejection technique according to the present invention;

FIG. 11 is a graph illustrating an improved noise artifact rejectiontechnique according to the present invention;

FIG. 12 is a graph illustrating a plot of a neuromuscular response (EMG)over time (in response to a stimulus current pulse) showing the mannerin which voltage extrema (_(VMax or Min)), (V_(Min or Max)) occur attimes T1 and T2, respectively;

FIG. 13 is a graph illustrating a histogram as may be employed as partof a T1, T2 artifact rejection technique according to an alternateembodiment of the present invention;

FIGS. 14A-14E are graphs illustrating a current threshold-huntingalgorithm according to one embodiment of the present invention;

FIG. 15 is a series of graphs illustrating a multi-channel currentthreshold-hunting algorithm according to one embodiment of the presentinvention;

FIGS. 16-19 are top views of a neurophysiology-based surgical accesssystem according to one embodiment of the present invention in useaccessing a surgical target site in the spine;

FIG. 20 is an exemplary screen display illustrating one embodiment ofthe nerve proximity or detection feature of the surgical access systemof the present invention;

FIG. 21 is an exemplary screen display illustrating one embodiment ofthe nerve detection feature of the surgical access system of the presentinvention;

FIG. 22 is a graph illustrating a method of determining the direction ofa nerve (denoted as an “octagon”) relative to an instrument having four(4) orthogonally disposed stimulation electrodes (denoted by the“circles”) according to one embodiment of the present invention;

FIGS. 23-24 are exemplary screen displays illustrating one embodiment ofthe pedicle integrity assessment feature of the present invention;

FIGS. 25-27 are exemplary screen displays illustrating anotherembodiment of the pedicle integrity assessment feature of the presentinvention;

FIG. 28 is a graph illustrating recruitment curves for a generallyhealthy nerve (denoted “A”) and a generally unhealthy nerve (denoted“B”) according to the nerve pathology monitoring feature of the presentinvention;

FIGS. 29-30 are perspective and side views, respectively, of anexemplary nerve root retractor assembly according to one embodiment ofthe present invention;

FIG. 31 is a perspective view of an exemplary nerve root retractoraccording to one embodiment of the present invention;

FIG. 32 is an exemplary screen display illustrating one embodiment ofthe neural pathology monitoring feature of the present invention,specifically for monitoring change in nerve function of a healthy nervedue to nerve retraction;

FIG. 33 is an exemplary screen display illustrating another embodimentof the neural pathology monitoring feature of the present invention,specifically for monitoring change in nerve function of a healthy nervedue to nerve retraction;

FIG. 34 is an exemplary screen display illustrating one embodiment ofthe neural pathology monitoring feature of the present invention,specifically for monitoring change in nerve function of an unhealthynerve due to the performance of a surgical procedure; and

FIG. 35 is an exemplary screen display illustrating another embodimentof the neural pathology monitoring feature of the present invention,specifically for monitoring change in nerve function of an unhealthynerve due to the performance of a surgical procedure.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

The present invention is capable of performing a variety of surgicalprocedures and assessments by combining neurophysiology monitoring withany of a variety of instruments used in or in preparation for surgery(referred to herein as “surgical accessories”). By way of example only,such surgical accessories may include, but are not necessarily limitedto, any number of devices or components for creating an operativecorridor to a surgical target site (such as K-wires, sequentiallydilating cannula systems, distractor systems, and/or retractor systems),for retracting or otherwise protecting a nerve root before, duringand/or after surgery (such as a nerve root retractor), and/or forassessing pedicle integrity (such as a pedicle screw test probe).Although described herein largely in terms of use in spinal surgery, itis to be readily appreciated that the teachings of the method andapparatus of the present invention are suitable for use in any number ofadditional surgical procedures wherein tissue having significant neuralstructures must be passed through (or near) in order to establish anoperative corridor to a surgical target site, wherein neural structuresare retracted, and/or wherein neural structures are located adjacentbony structures.

FIG. 1 illustrates the fundamental method steps according to the presentinvention, namely: (step 10) stimulating one or more electrodes providedon a surgical accessory; (step 12) measuring the response of nervesinnervated by the stimulation of step 10; (step 14) determining arelationship between the surgical accessory and the nerve based upon theresponse measured in step 12; and (step 16) communicating thisrelationship to the surgeon in an easy-to-interpret fashion.

The step of stimulating may be accomplished by applying any of a varietyof suitable stimulation signals to the electrode(s) on the surgicalaccessory, including voltage and/or current pulses of varying magnitudeand/or frequency. The stimulating step may be performed at differenttimes depending upon the particular surgical accessory in question. Forexample, when employed with a surgical access system, stimulation step10 may be performed during and/or after the process of creating anoperative corridor to the surgical target site. When used for pedicleintegrity assessments, stimulation step 10 may be performed before,during and/or after the formation of the hole established to receive apedicle screw, as well as before, during and/or after the pedicle screwis introduced into the hole. With regard to neural pathology monitoring,stimulation step 10 may be performed before, during and/or afterretraction of the nerve root.

The step of measuring the response of nerves innervated by thestimulation step 10 may be performed in any number of suitable fashions,including but not limited to the use of evoked muscle action potential(EMAP) monitoring techniques (that is, measuring the EMG responses ofmuscle groups associated with a particular nerve). According to oneaspect of the present invention, the measuring step is preferablyaccomplished via monitoring or measuring the EMG responses of themuscles innervated by the nerve(s) stimulated in step 10 for each of thepreferred functions of the present invention: surgical access, pedicleintegrity assessments, and neural pathology monitoring.

The step of determining a relationship between the surgical accessoryand the nerve based upon the measurement step 12 may be performed in anynumber of suitable fashions depending upon the manner of measuring theresponse of step 12, and may define the relationship in any of a varietyof fashions (based on any number of suitable parameters and/orcharacteristics). By way of example only, the step 14 of determining arelationship, within the context of a surgical access system, mayinvolve identifying when (and preferably the degree to which) thesurgical accessory comes into close proximity with a given nerve (“nerveproximity”) and/or identifying the relative direction between thesurgical accessory and the nerve (“nerve direction”). For a pedicleintegrity assessment, the relationship between the surgical accessory(screw test probe) and the nerve is whether electrical communication isestablished therebetween. If electrical communication is established,this indicates that the medial wall of the pedicle has been cracked,stressed, or otherwise breached during the steps of hole formationand/or screw introduction. If not, this indicates that the integrity ofthe medial wall of the pedicle has remained intact during hole formationand/or screw introduction. This characteristic is based on theinsulating properties of bone. For neural pathology assessmentsaccording to the present invention, the step 14 relationship may be, byway of example only, whether the neurophysiologic response of the nervehas changed over time. Such neurophysiologic responses may include, butare not necessarily limited to, the onset stimulation threshold for thenerve in question, the slope of the response vs. the stimulation signalfor the nerve in question and/or the saturation level of the nerve inquestion. Changes in these parameters will indicate if the health orstatus of the nerve is improving or deteriorating, such as may resultduring surgery.

The step of communicating this relationship to the surgeon in aneasy-to-interpret fashion may be accomplished in any number of suitablefashions, including but not limited to the use of visual indicia (suchas alpha-numeric characters, light-emitting elements, and/or graphics)and audio communications (such as a speaker element). By way of exampleonly, with regard to surgical access systems, step 16 of communicatingthe relationship may include, but is not necessarily limited to,visually representing the stimulation threshold of the nerve (indicatingrelative distance or proximity to the nerve), providing color codedgraphics to indicate general proximity ranges (i.e. “green” for a rangeof stimulation thresholds above a predetermined safe value, “red” forrange of stimulation thresholds below a predetermined unsafe value, and“yellow” for the range of stimulation thresholds in between thepredetermined safe and unsafe values—designating caution), as well asproviding an arrow or other suitable symbol for designating the relativedirection to the nerve. This is an important feature of the presentinvention in that, by providing such proximity and directioninformation, a user will be kept informed as to whether a nerve is tooclose to a given surgical accessory element during and/or after theoperative corridor is established to the surgical target site. This isparticularly advantageous during the process of accessing the surgicaltarget site in that it allows the user to actively avoid nerves andredirect the surgical access components to successfully create theoperative corridor without impinging or otherwise compromising thenerves. Based on these nerve proximity and direction features, then, thepresent invention is capable of passing through virtually any tissuewith minimal (if at all) risk of impinging or otherwise damagingassociated neural structures within the tissue, thereby making thepresent invention suitable for a wide variety of surgical applications.

With regard to pedicle integrity assessments, the step 16 ofcommunicating the relationship may include, but is not necessarilylimited to, visually representing the actual stimulation threshold of anexiting nerve root alone or in combination with the stimulationthreshold of a bare nerve root (with or without the differencetherebetween), as well as with providing color coded graphics toindicate general ranges of pedicle integrity (i.e. “green” for a rangeof stimulation thresholds above a predetermined safe value--indicating“breach unlikely”, “red” for range of stimulation thresholds below apredetermined unsafe value—indicating “breach likely”, and “yellow” forthe range of stimulation thresholds between the predetermined safe andunsafe values—indicating “possible breach”). This is a significantfeature, and advantage over the prior art, in that it provides astraightforward and easy to interpret representation as to whether apedicle has been breached during and/or after the process of forming thehole and/or introducing the pedicle screw. Identifying such a potentialbreach is helpful in that it prevents or minimizes the chance that amisplaced pedicle screw (that is, one breaching a wall of the pedicle,such as, by way of example, the medial wall) will be missed until afterthe surgery. Instead, any such misplaced pedicle screws, when stimulatedaccording to the present invention, will produce an EMG response at amyotome level associated with the nerve in close proximity to thepedicle screw that is breaching the pedicle wall. This will indicate tothe surgeon that the pedicle screw needs to be repositioned. But forthis system and technique, patients may be released and subsequentlyexperience pain due to the contact between the exiting nerve root andthe pedicle screw, which oftentimes requires another costly and painfulsurgery.

As for neural pathology monitoring, the step 16 of communicating therelationship may include, but is not necessarily limited to, visuallyrepresenting the changes over time in the onset stimulation threshold ofthe nerve, the slope of the response versus the stimulation threshold ofthe nerve and/or the saturation level of the nerve. Once again, thesechanges may indicate if the health or status of the nerve is improvingor deteriorating, such as may result during surgery and/or retraction.This feature is important in that it may provide qualitative feedback onthe effect of the particular surgery. If it appears the health or status(pathology) of the nerve is deteriorating over time, the user may beinstructed to stop or lessen the degree of retraction to avoid suchdeterioration. If the pathology of the nerve improves over time, it mayindicate the success of the surgery in restoring or improving nervefunction, such as may be the case in decompressive spinal surgery.

FIGS. 2-3 illustrate, by way of example only, a surgical system 20provided in accordance with a broad aspect of the present invention. Thesurgical system 20 includes a control unit 22, a patient module 24, anEMG harness 26 and return electrode 28 coupled to the patient module 24,and a host of surgical accessories 30 capable of being coupled to thepatient module 24 via one or more accessory cables 32. In the embodimentshown, the surgical accessories 30 include (by way of example only) asequential dilation access system 34, a pedicle testing assembly 36, anda nerve root retractor assembly 38. The control unit 22 includes a touchscreen display 40 and a base 42, which collectively contain theessential processing capabilities for controlling the surgical system20. The patient module 24 is connected to the control unit 22 via a datacable 44, which establishes the electrical connections andcommunications (digital and/or analog) between the control unit 22 andpatient module 24. The main functions of the control unit 22 includereceiving user commands via the touch screen display 40, activatingstimulation in the requested mode (nerve proximity, nerve direction,screw test, and nerve pathology), processing signal data according todefined algorithms (described below), displaying received parameters andprocessed data, and monitoring system status and report faultconditions. The touch screen display 40 is preferably equipped with agraphical user interface (GUI) capable of communicating information tothe user and receiving instructions from the user. The display 40 and/orbase 42 may contain patient module interface circuitry that commands thestimulation sources, receives digitized signals and other informationfrom the patient module 24, processes the EMG responses to extractcharacteristic information for each muscle group, and displays theprocessed data to the operator via the display 40.

As will be described in greater detail below, the surgical system 20 iscapable of performing one or more of the following functions: (1)determination of nerve proximity and/or nerve direction relative to thesequential dilation access system 34 during and following the creationof an operative corridor to surgical target site; (2) assessment ofpedicle integrity after hole formation and/or after pedicle screwplacement via the pedicle testing assembly 36; and/or (3) assessment ofnerve pathology (health or status) before, during, and/or after asurgical procedure via the nerve root retractor assembly 38. Surgicalsystem 20 accomplishes this by having the control unit 22 and patientmodule 24 cooperate to send stimulation signals to one or morestimulation electrodes on the various surgical accessories 30. Dependingupon the location of the surgical accessories within a patient, thestimulation signals may cause nerves adjacent to or in the generalproximity of the surgical accessories 30 to innervate, which, in turn,can be monitored via the EMG harness 26. The nerve proximity anddirection, pedicle integrity, and nerve pathology features of thepresent invention are based on assessing the evoked response of thevarious muscle myotomes monitored by the surgical system 20 via EMGharness 26.

The sequential dilation access system 34 comprises, by way of exampleonly, a K-wire 46, one or more dilating cannula 48, and a workingcannula 50. As will be explained in greater detail below, thesecomponents 46-50 are designed to bluntly dissect the tissue between thepatient's skin and the surgical target site. In an important aspect ofthe present invention, the K-wire 46, dilating cannula 48 and/or workingcannula 50 may be equipped with one or more stimulation electrodes todetect the presence and/or location of nerves in between the skin of thepatient and the surgical target site. To facilitate this, a surgicalhand-piece 52 is provided for electrically coupling the surgicalaccessories 46-50 to the patient module 24 (via accessory cable 32). Ina preferred embodiment, the surgical hand piece 42 includes one or morebuttons for selectively initiating the stimulation signal (preferably, acurrent signal) from the control unit 12 to a particular surgical accesscomponent 46-50. Stimulating the electrode(s) on these surgical accesscomponents 46-50 during passage through tissue in forming the operativecorridor will cause nerves that come into close or relative proximity tothe surgical access components 46-50 to depolarize, producing a responsein the innervated myotome. By monitoring the myotomes associated withthe nerves (via the EMG harness 26 and recording electrode 27) andassessing the resulting EMG responses (via the control unit 22), thesequential dilation access system 34 is capable of detecting thepresence (and optionally direction to) such nerves, thereby providingthe ability to actively negotiate around or past such nerves to safelyand reproducibly form the operative corridor to a particular surgicaltarget site. In one embodiment, the sequential dilation access system 34is particularly suited for establishing an operative corridor to anintervertebral target site in a postero-lateral, trans-psoas fashion soas to avoid the bony posterior elements of the spinal column.

The pedicle testing assembly 36 includes a surgical accessory handleassembly 54 and a pedicle probe 56. The handle assembly 54 includes acable 55 for establishing electrical communication with the patientmodule 24 (via the accessory cable 32). In a preferred embodiment, thepedicle probe 56 may be selectively removed from the handle assembly 54,such as by unscrewing a threaded cap 58 provided on the distal end ofthe handle assembly 54 (through which the proximal end of the pedicleprobe 56 passes). The pedicle probe 56 includes a ball-tipped distal end60 suitable for introduction into a pedicle hole (after hole formationbut before screw insertion) and/or for placement on the head of a fullyintroduced pedicle screw. In both situations, the user may operate oneor more buttons of the handle assembly 54 to selectively initiate astimulation signal (preferably, a current signal) from the patientmodule 24 to the pedicle probe 56. With the pedicle probe 56 touchingthe inner wall of the pedicle hole and/or the fully introduced pediclescrew, applying a stimulation signal in this fashion serves to test theintegrity of the medial wall of the pedicle. That is, a breach orcompromise in the integrity of the pedicle will allow the stimulationsignal to pass through the pedicle and innervate an adjacent nerve root.By monitoring the myotomes associated with the nerve roots (via the EMGharness 26 and recording electrode 27) and assessing the resulting EMGresponses (via the control unit 22), the surgical system 20 can assesswhether a pedicle breach occurred during hole formation and/or screwintroduction. If a breach or potential breach is detected, the user maysimply withdraw the misplaced pedicle screw and redirect to ensureproper placement.

The nerve root retractor assembly 38, in a preferred embodiment,comprises the same style surgical accessory handle assembly 54 asemployed with in the pedicle testing assembly 36, with a selectivelyremovable nerve root retractor 62. The nerve root retractor 62 has agenerally angled orientation relative to the longitudinal axis of thehandle assembly 54, and includes a curved distal end 64 having agenerally arcuate nerve engagement surface 66 equipped with one or morestimulation electrodes (not shown). In use, the nerve root retractor 62is introduced into or near a surgical target site in order to hook andretract a given nerve out of the way. According to the presentinvention, the nerve root may be stimulated (monopolar or bipolar)before, during, and/or after retraction in order to assess the degree towhich such retraction impairs or otherwise degrades nerve function overtime. To do so, the user may operate one or more buttons of the handleassembly 54 to selectively transmit a stimulation signal (preferably, acurrent signal) from the patient module 24 to the electrode(s) on theengagement surface 66 of the nerve root retractor 62. By monitoring themyotome associated with the nerve root being retracted (via the EMGharness 26) and assessing the resulting EMG responses (via the controlunit 22), the surgical system 20 can assess whether (and the degree towhich) such retraction impairs or adversely affects nerve function overtime. With this information, a user may wish to periodically release thenerve root from retraction to allow nerve function to recover, therebypreventing or minimizing the risk of long-term or irreversible nerveimpairment. As will be described in greater detail below, a similarneural pathology assessment can be undertaken, whereby an unhealthynerve may be monitored to determine if nerve function improves due to aparticular surgical procedure, such as spinal nerve decompressionsurgery.

A discussion of the algorithms and principles behind the neurophysiologyfor accomplishing these functions will now be undertaken, followed by adetailed description of the various implementations of these principlesaccording to the present invention.

FIGS. 4 and 5 illustrate a fundamental aspect of the present invention:a stimulation signal (FIG. 4) and a resulting evoked response (FIG. 5).By way of example only, the stimulation signal is preferably astimulation current signal (I_(Stim)) having rectangular monophasicpulses with a frequency and amplitude adjusted by system software. In astill further preferred embodiment, the stimulation current (I_(Stim))may be coupled in any suitable fashion (i.e. AC or DC) and comprisesrectangular monophasic pulses of 200 microsecond duration. The amplitudeof the current pulses may be fixed, but will preferably sweep fromcurrent amplitudes of any suitable range, such as from 2 to 100 mA. Foreach nerve and myotome there is a characteristic delay from thestimulation current pulse to the EMG response (typically between 5 to 20ms). To account for this, the frequency of the current pulses is set ata suitable level such as, in a preferred embodiment, 4 Hz to 10 Hz (andmost preferably 4.5 Hz), so as to prevent stimulating the nerve beforeit has a chance to recover from depolarization. The EMG response shownin FIG. 5 can be characterized by a peak-to-peak voltage ofV_(pp)=V_(max)−V_(min).

FIG. 6 illustrates an alternate manner of setting the maximumstimulation frequency, to the extent it is desired to do so rather thansimply selecting a fixed maximum stimulation frequency (such as 4.5 Hz)as described above. According to this embodiment, the maximum frequencyof the stimulation pulses is automatically adjusted. After eachstimulation, F_(max) will be computed as:F_(max)=1/(T2+T_(Safety Margin)) for the largest value of T2 from eachof the active EMG channels. In one embodiment, the Safety Margin is 5ms, although it is contemplated that this could be varied according toany number of suitable durations. Before the specified number ofstimulations, the stimulations will be performed at intervals of 100-120ms during the bracketing state, intervals of 200-240 ms during thebisection state, and intervals of 400-480 ms during the monitoringstate. After the specified number of stimulations, the stimulations willbe performed at the fastest interval practical (but no faster thanF_(max)) during the bracketing state, the fastest interval practical(but no faster than Fmax/2) during the bisection state, and the fastestinterval practical (but no faster than Fmax/4) during the monitoringstate. The maximum frequency used until F_(max) is calculated ispreferably 10 Hz, although slower stimulation frequencies may be usedduring some acquisition algorithms. The value of F_(max) used isperiodically updated to ensure that it is still appropriate. Forphysiological reasons, the maximum frequency for stimulation will be seton a per-patient basis. Readings will be taken from all myotomes and theone with the slowest frequency (highest T2) will be recorded

A basic premise behind the neurophysiology employed in the presentinvention is that each nerve has a characteristic threshold currentlevel (I_(Thresh)) at which it will depolarize. Below this threshold,current stimulation will not evoke a significant EMG response (V_(pp)).Once the stimulation threshold (I_(Thresh)) is reached, the evokedresponse is reproducible and increases with increasing stimulation untilsaturation is reached. This relationship between stimulation current andEMG response may be represented graphically via a so-called “recruitmentcurve,” such as shown in FIG. 7, which includes an onset region, alinear region, and a saturation region. By way of example only, thepresent invention defines a significant EMG response to have a V_(pp) ofapproximately 100 uV. In a preferred embodiment, the lowest stimulationcurrent that evokes this threshold voltage (V_(Thresh)) is calledI_(thresh). As will be described in greater detail below, changes in thecurrent threshold (I_(thresh)) over time may indicate that the relativedistance between the nerve and the stimulation electrode is changing(indicating nerve migration towards the surgical accessory having thestimulation electrode and/or movement of the surgical accessory towardsthe nerve). This is useful in performing proximity assessments betweenthe electrode and the nerve according to an aspect of the presentinvention. Changes in the current threshold (I_(thresh)) may also beindicative of a change in the degree of electrical communication betweena stimulation electrode and a nerve. This may be helpful, by way ofexample, in assessing if a screw or similar instrument has inadvertentlybreached the medial wall of a pedicle. More specifically, where aninitial determination of (I_(thresh)), such as by applying a stimulationcurrent to the interior of a hole created to receive a pedicle screw, isgreater than a later determination of (I_(thresh)), such as by applyinga stimulation current to the tip of the pedicle screw after insertion,the decrease in I_(thresh), if large enough, may indicate electricalcommunication between the pedicle screw and the nerve. Based on theinsulation properties of bone, such electrical communication wouldindicate a breach of the pedicle. As will also be in greater detailbelow, changes in the current threshold (I_(thresh)), the slope of thelinear region, and the saturation level over time are indicative ofchanges in the pathology (that is, health or status) of a given nerve.This is useful in assessing the effects of surgery on an unhealthy nerve(such as decompression surgery) as well as assessing the effects ofnerve retraction on a healthy nerve (so as to prevent or minimize therisk of damage due to retraction).

In order to obtain this useful information, the present invention mustfirst identify the peak-to-peak voltage (V_(pp)) of each EMG responsecorresponding a given stimulation current (I_(Stim)). The existencestimulation and/or noise artifacts, however, can conspire to create anerroneous V_(pp) measurement of the electrically evoked EMG response. Toovercome this challenge, the surgical system 20 of the present inventionmay employ any number of suitable artifact rejection techniques,including the traditional stimulation artifact rejection technique shownin FIG. 8. Under this technique, stimulation artifact rejection isundertaken by providing a simple artifact rejection window T1 _(WIN) atthe beginning of the EMG waveform. During this T1 window, the EMGwaveform is ignored and V_(pp) is calculated based on the max and minvalues outside this window. (T1 is the time of the first extremum (minor max) and T2 is the time of the second extremum.) In one embodiment,the artifact rejection window T1 _(WIN) may be set to about 7.3 msec.While generally suitable, there are situations where this stimulationartifact rejection technique of FIG. 8 is not optimum, such as in thepresence of a large stimulation artifact (see FIG. 9). The presence of alarge stimulation artifact causes the stimulation artifact to cross overthe window T1 _(WIN) and blend in with the EMG. Making the stimulationartifact window larger is not effective, since there is no clearseparation between EMG and stimulation artifact.

FIG. 10 illustrates a stimulation artifact rejection technique accordingto the present invention, which solves the above-identified problem withtraditional stimulation artifact rejection. Under this technique, a T1validation window (T1-V_(WIN)) is defined immediately following the T1window (T1 _(WIN)). If the determined V_(pp) exceeds the threshold forrecruiting, but T1 falls within this T1 validation window, then thestimulation artifact is considered to be substantial and the EMG isconsidered to have not recruited. An operator may be alerted, based onthe substantial nature of the stimulation artifact. This method ofstimulation artifact rejection is thus able to identify situations wherethe stimulation artifact is large enough to cause the V_(pp) to exceedthe recruit threshold. To account for noise, the T1 validation window(T1-V_(WIN)) should be within the range of 0.1 ms to 1 ms wide(preferably about 0.5 ms). The T1 validation window (T1-V_(WIN)) shouldnot be so large that the T1 from an actual EMG waveform could fallwithin.

FIG. 11 illustrates a noise artifact rejection technique according tothe present invention. When noise artifacts fall in the time windowwhere an EMG response is expected, their presence can be difficult toidentify. Artifacts outside the expected response window, however, arerelatively easy to identify. The present invention capitalizes on thisand defines a T2 validation window (T2-V_(WIN)) analogous to the T1validation window (T1-V_(WIN)) described above with reference to FIG.10. As shown, T2 must occur prior to a defined limit, which, accordingto one embodiment of the present invention, may be set having a range ofbetween 40 ms to 50 ms (preferably about 47 ms). If the V_(pp) of theEMG response exceeds the threshold for recruiting, but T2 falls beyondthe T2 validation window (T2-V_(WIN)), then the noise artifact isconsidered to be substantial and the EMG is considered to have notrecruited. An operator may be alerted, based on the substantial natureof the noise artifact.

FIG. 12 illustrates a still further manner of performing stimulationartifact rejection according to an alternate embodiment of the presentinvention. This artifact rejection is premised on the characteristicdelay from the stimulation current pulse to the EMG response. For eachstimulation current pulse, the time from the current pulse to the firstextremum (max or min) is T1 and to the second extremum (max or min) isT2. As will be described below, the values of T₁, T₂ are each compiledinto a histogram period (see FIG. 13). New values of T₁, T₂ are acquiredfor each stimulation and the histograms are continuously updated. Thevalue of T₁, and T₂ used is the center value of the largest bin in thehistogram. The values of T₁, T₂ are continuously updated as thehistograms change. Initially V_(pp) is acquired using a window thatcontains the entire EMG response. After 20 samples, the use of T₁, T₂windows is phased in over a period of 200 samples. V_(max) and V_(min)are then acquired only during windows centered around T₁, T₂ with widthsof, by way of example only, 5 msec. This method of acquiring V_(pp)automatically rejects the artifact if T₁, T₂ fall outside of theirrespective windows.

Having measured each V_(pp) EMG response (as facilitated by thestimulation and/or noise artifact rejection techniques described above),this V_(pp) information is then analyzed relative to the stimulationcurrent in order to determine a relationship between the nerve and thegiven surgical accessory transmitting the stimulation current. Morespecifically, the present invention determines these relationships(between nerve and surgical accessory) by identifying the minimumstimulation current (I_(Thresh)) capable of resulting in a predeterminedV_(pp) EMG response. According to the present invention, thedetermination of I_(Thresh) may be accomplished via any of a variety ofsuitable algorithms or techniques.

FIGS. 14A-14E illustrate, by way of example only, a threshold-huntingalgorithm for quickly finding the threshold current (I_(Thresh)) foreach nerve being stimulated by a given stimulation current (I_(Stim)).Threshold current (I_(Thresh)), once again, is the minimum stimulationcurrent (I_(Stim)) that results in a V_(pp) that is greater than a knownthreshold voltage (V_(Thresh)). The value of is adjusted by a bracketingmethod as follows. The first bracket is 0.2 mA and 0.3 mA. If the V_(pp)corresponding to both of these stimulation currents is lower thanV_(Thresh), then the bracket size is doubled to 0.2 mA and 0.4 mA. Thisdoubling of the bracket size continues until the upper end of thebracket results in a V_(pp) that is above V_(Thresh). The size of thebrackets is then reduced by a bisection method. A current stimulationvalue at the midpoint of the bracket is used and if this results in aV_(pp) that is above V_(Thresh), then the lower half becomes the newbracket. Likewise, if the midpoint V_(pp) is below V_(Thresh) then theupper half becomes the new bracket. This bisection method is used untilthe bracket size has been reduced to I_(Thresh) mA. I_(Thresh) may beselected as a value falling within the bracket, but is preferablydefined as the midpoint of the bracket.

The threshold-hunting algorithm of this embodiment will support threestates: bracketing, bisection, and monitoring. A stimulation currentbracket is a range of stimulation currents that bracket the stimulationcurrent threshold I_(Thresh). The width of a bracket is the upperboundary value minus the lower boundary value. If the stimulationcurrent threshold I_(Thresh) of a channel exceeds the maximumstimulation current, that threshold is considered out-of-range. Duringthe bracketing state, threshold hunting will employ the method below toselect stimulation currents and identify stimulation current bracketsfor each EMG channel in range.

The method for finding the minimum stimulation current uses the methodsof bracketing and bisection. The “root” is identified for a functionthat has the value −1 for stimulation currents that do not evokeadequate response; the function has the value +1 for stimulationcurrents that evoke a response. The root occurs when the function jumpsfrom −1 to +1 as stimulation current is increased: the function neverhas the value of precisely zero. The root will not be known exactly, butonly with a level of precision related to the minimum bracket width. Theroot is found by identifying a range that must contain the root. Theupper bound of this range is the lowest stimulation current I_(Thresh)where the function returns the value +1, i.e. the minimum stimulationcurrent that evokes response. The lower bound of this range is thehighest stimulation current I_(Thresh) where the function returns thevalue −1, i.e. the maximum stimulation current that does not evoke aresponse.

The proximity function begins by adjusting the stimulation current untilthe root is bracketed (FIG. 14B). The initial bracketing range may beprovided in any number of suitable ranges. In one embodiment, theinitial bracketing range is 0.2 to 0.3 mA. If the upper stimulationcurrent does not evoke a response, the upper end of the range should beincreased. The range scale factor is 2. The stimulation current shouldpreferably not be increased by more than 10 mA in one iteration. Thestimulation current should preferably never exceed the programmedmaximum stimulation current. For each stimulation, the algorithm willexamine the response of each active channel to determine whether itfalls within that bracket. Once the stimulation current threshold ofeach channel has been bracketed, the algorithm transitions to thebisection state.

During the bisection state (FIGS. 14C and 14D), threshold hunting willemploy the method described below to select stimulation currents andnarrow the bracket to a selected width (for example, 0.1 mA) for eachEMG channel with an in-range threshold. After the minimum stimulationcurrent has been bracketed (FIG. 14B), the range containing the root isrefined until the root is known with a specified accuracy. The bisectionmethod is used to refine the range containing the root. In oneembodiment, the root should be found to a precision of 0.1 mA. Duringthe bisection method, the stimulation current at the midpoint of thebracket is used. If the stimulation evokes a response, the bracketshrinks to the lower half of the previous range. If the stimulationfails to evoke a response, the bracket shrinks to the upper half of theprevious range. The proximity algorithm is locked on the electrodeposition when the response threshold is bracketed by stimulationcurrents separated by the selected width (i.e. 0.1 mA). The process isrepeated for each of the active channels until all thresholds areprecisely known. At that time, the algorithm enters the monitoringstate.

During the monitoring state (FIG. 14E), threshold hunting will employthe method described below to select stimulation currents and identifywhether stimulation current thresholds are changing. In the monitoringstate, the stimulation current level is decremented or incremented by0.1 mA, depending on the response of a specific channel. If thethreshold has not changed then the lower end of the bracket should notevoke a response, while the upper end of the bracket should. If eitherof these conditions fail, the bracket is adjusted accordingly. Theprocess is repeated for each of the active channels to continue toassure that each threshold is bracketed. If stimulations fail to evokethe expected response three times in a row, then the algorithm maytransition back to the bracketing state in order to reestablish thebracket.

When it is necessary to determine the stimulation current thresholds(I_(Thresh)) for more than one channel, they will be obtained bytime-multiplexing the threshold-hunting algorithm as shown in FIG. 15.During the bracketing state, the algorithm will start with a stimulationcurrent bracket of 0.2 mA and increase the size of the bracket. Witheach bracket, the algorithm will measure the V_(pp) of all channels todetermine which bracket they fall into. After this first pass, thealgorithm will determine which bracket contains the I_(Thresh) for eachchannel. Next, during the bisection state, the algorithm will start withthe lowest bracket that contains an I_(Thresh) and bisect it untilI_(Thresh) is found within 0.1 mA. If there are more than one I_(Thresh)within a bracket, they will be separated out during the bisectionprocess, and the one with the lowest value will be found first. Duringthe monitoring state, the algorithm will monitor the upper and lowerboundaries of the brackets for each I_(Thresh), starting with thelowest. If the I_(Thresh) for one or more channels is not found in it'sbracket, then the algorithm goes back to the bracketing state tore-establish the bracket for those channels.

A still further manner of performing multi-channel threshold hunting isdescribed as follows, with reference to FIGS. 14-15. This techniquemonitors multiple channels but reports the result for a single channel.The user chooses one of two channel selection modes: auto or manual. Inthe manual channel selection mode, the system will track the stimulationthreshold I_(Thresh) for a single EMG channel, as shown in FIG. 14. Inthe auto channel selection mode, the system will monitor responses on aset of channels and track to the lowest responding channel. The automode permits the user to select the set of channels to track. Individualchannels can be added or subtracted from the set at any time. Trackingto the lowest responding channel is performed in this fashion. First,after stimulation, if no channels in the selected set respond, then thestimulation current is below the lowest responding channel. If anychannels respond, then the stimulation current is above the lowestresponding channel. Coupling this logic with the bracketing, bisection,and monitoring technique described above allows the system to track tothe lowest responding channel, and do so in a quick and accuratefashion.

If during monitoring, the tracked channel falls out of the bracket, orif any channel responds at the low end of the bracket, then the bracketwill be expanded again, as before, until the lowest responding channelis bracketed again. However, unlike the embodiments shown in FIGS. 14and 15, the bracket is expanded in situ rather than beginning again fromthe start. For example, a bracket of 4.5 to 4.6 mA that fails to recruitat both levels is expanded to higher currents. First, the bracket widthis doubled from 0.1 mA to 0.2 mA, resulting in stimulation current at4.7 mA. If this fails to recruit, the bracket is again doubled to 0.4mA, with stimulation current at 4.9 mA. The pattern continues withstimulations at 5.3, 6.1, and 9.3 mA, corresponding to bracket sizes of0.8, 1.6, and 3.2 mA, until the threshold is bracketed. If a response isevoked at both ends of the original bracket, the same bracket-doublingtechnique is used moving toward lower stimulation currents.

The reason for doubling the bracket size each time is to identify thethreshold current with as few stimulations as practical. The reason forstarting the bracket doubling in situ rather than starting over fromzero is twofold: (1) to take advantage of threshold information that isalready known, and (2) it is more likely that the current threshold hasnot moved far from where it was previously bracketed. The advantage oftracking only to the lowest channel is that it provides the mostrelevant nerve proximity information with fewer stimulation pulses thanmulti-channel detection as with that shown in FIG. 15. This is anadvantage because fewer stimulation pulses means a faster respondingsystem, with the goal being to be able to track movement of thestimulation electrode in real time.

After identifying the threshold current I_(Thresh), this information maybe employed to determine any of a variety of relationships between thesurgical accessory and the nerve. For example, as will be described ingreater detail below, determining the current threshold I_(Thresh) of anerve while using a surgical access system (such as the sequentialdilation system 34 of FIG. 2) may involve determining when (andpreferably the degree to which) the surgical accessory comes into closeproximity with a given nerve (“nerve proximity”) and/or identifying therelative direction between the surgical accessory and the nerve (“nervedirection”). For a pedicle integrity assessment, the relationshipbetween the pedicle testing assembly 36 and the nerve is whetherelectrical communication is established therebetween. If electricalcommunication is established, this indicates that the medial wall of thepedicle has been cracked, stressed, or otherwise breached during thesteps of hole formation and/or screw introduction. If not, thisindicates that the integrity of the medial wall of the pedicle hasremained intact during hole formation and/or screw introduction. Thischaracteristic is based on the insulating properties of bone. For neuralpathology assessments according to the present invention, therelationship may be, by way of example only, whether theneurophysiologic response of the nerve has changed over time. Suchneurophysiologic responses may include, but are not necessarily limitedto, the onset stimulation threshold for the nerve in question, the slopeof the response vs. the stimulation signal for the nerve in questionand/or the saturation level of the nerve in question. Changes in theseparameters will indicate if the health or status of the nerve isimproving or deteriorating, such as may result during surgery or nerveretraction.

In a significant aspect of the present invention, the relationshipsdetermined above based on the current threshold determination may becommunicated to the user in an easy to use format, including but notlimited to, alpha-numeric and/or graphical information regarding mode ofoperation, nerve proximity, nerve direction, nerve pathology, pedicleintegrity assessments, stimulation level, EMG responses, advance or holdinstructions, instrument in use, set-up, and related instructions forthe user. This advantageously provides the ability to present simplifiedyet meaningful data to the user, as opposed to the actual EMG waveformsthat are displayed to the users in traditional EMG systems. Due to thecomplexity in interpreting EMG waveforms, such prior art systemstypically require an additional person specifically trained in suchmatters which, in turn, can be disadvantageous in that it translatesinto extra expense (having yet another highly trained person inattendance) and oftentimes presents scheduling challenges because mosthospitals do not retain such personnel.

Having described the fundamental aspects of the neurophysiologyprinciples and algorithms of the present invention, variousimplementations according to the present invention will now bedescribed.

I. Surgical Access: Nerve Proximity and Direction

FIGS. 2-3 illustrate an exemplary embodiment of the surgical system 20of the present invention, including the sequential dilation accesssystem 34. The sequential dilation access system 34 of the presentinvention is capable of accomplishing safe and reproducible access to asurgical target site. It does so by detecting the existence of (andoptionally the distance and/or direction to) neural structures before,during, and after the establishment of an operative corridor through (ornear) any of a variety of tissues having such neural structures, which,if contacted or impinged, may otherwise result in neural impairment forthe patient. The surgical system 20 does so by electrically stimulatingnerves via one or more stimulation electrodes at the distal end of thesurgical access components 46-50 while monitoring the EMG responses ofthe muscle groups innervated by the nerves.

In one embodiment, the surgical system 20 accomplishes this through theuse of the surgical hand-piece 52, which may be electrically coupled tothe K-wire 46 via a first cable connector 51 a, 51 b and to either thedilating cannula 48 or the working cannula 50 via a second cableconnector 53 a, 53 b. For the K-wire 46 and working cannula 50, cablesare directly connected between these accessories and the respectivecable connectors 51 a, 53 a for establishing electrical connection tothe stimulation electrode(s). In one embodiment, a pincher or clamp-typedevice 57 is provided to selectively establish electrical communicationbetween the surgical hand-piece 52 and the stimulation electrode(s) onthe distal end of the cannula 48. This is accomplished by providingelectrical contacts on the inner surface of the opposing arms formingthe clamp-type device 57, wherein the contacts are dimensioned to beengaged with electrical contacts (preferably in a male-female engagementscenario) provided on the dilating cannula 48 and working cannula 50.The surgical hand-piece 52 includes one or more buttons such that a usermay selectively direct a stimulation current signal from the controlunit 22 to the electrode(s) on the distal ends of the surgical accesscomponents 46-50. In an important aspect, each surgical access component46-50 is insulated along its entire length, with the exception of theelectrode(s) at their distal end (and, in the case of the dilatingcannula 48 and working cannula 50, the electrical contacts at theirproximal ends for engagement with the clamp 57). The EMG responsescorresponding to such stimulation may be monitored and assessedaccording to the present invention in order to provide nerve proximityand/or nerve direction information to the user.

When employed in spinal procedures, for example, such EMG monitoringwould preferably be accomplished by connecting the EMG harness 26 to themyotomes in the patient's legs corresponding to the exiting nerve rootsassociated with the particular spinal operation level. In a preferredembodiment, this is accomplished via 8 pairs of EMG electrodes 27 placedon the skin over the major muscle groups on the legs (four per side), ananode electrode 29 providing a return path for the stimulation current,and a common electrode 31 providing a ground reference to pre-amplifiersin the patient module 24. Although not shown, it will be appreciatedthat any of a variety of electrodes can be employed, including but notlimited to needle electrodes. The EMG responses measured via the EMGharness 26 provide a quantitative measure of the nerve depolarizationcaused by the electrical stimulus. By way of example, the placement ofEMG electrodes 27 may be undertaken according to the manner shown inTable 1 below for spinal surgery:

TABLE 1 Color Channel ID Myotome Spinal Level Blue Right 1 Right VastusMedialis L2, L3, L4 Violet Right 2 Right Tibialis Anterior L4, L5 GreyRight 3 Right Biceps Femoris L5, S1, S2 White Right 4 Right Gastroc.Medial S1, S2 Red Left 1 Left Vastus Medialis L2, L3, L4 Orange Left 2Left Tibialis Anterior L4, L5 Yellow Left 3 Left Biceps Femoris L5, S1,S2 Green Left 4 Left Gastroc. Medial S1, S2

FIGS. 16-19 illustrate the sequential dilation access system 34 of thepresent invention in use creating an operative corridor to anintervertebral disk. As shown in FIG. 16, an initial dilating cannula 48is advanced towards the target site with the K-wire 46 disposed withinan inner lumen within the dilating cannula 48. This may be facilitatedby first aligning the K-wire 46 and initial dilating cannula 48 usingany number of commercially available surgical guide frames. In oneembodiment, as best shown in the expanded insets A and B, the K-wire 46and initial dilating cannula 48 are each equipped with a singlestimulation electrode 70 to detect the presence and/or location ofnerves in between the skin of the patient and the surgical target site.More specifically, each electrode 70 is positioned at an angle relativeto the longitudinal axis of the K-wire 46 and dilator 48 (and workingcannula 50). In one embodiment, this angle may range from 5 to 85degrees from the longitudinal axis of these surgical access components46-50. By providing each stimulation electrode 70 in this fashion, thestimulation current will be directed angularly from the distal tip ofthe respective accessory 46, 48. This electrode configuration isadvantageous in determining proximity, as well as direction, accordingto the present invention in that a user may simply rotate the K-wire 46and/or dilating cannula 48 while stimulating the electrode 70. This maybe done continuously or step-wise, and preferably while in a fixed axialposition. In either case, the user will be able to determine thelocation of nerves by viewing the proximity information on the displayscreen 40 and observing changes as the electrode 70 is rotated. This maybe facilitated by placing a reference mark (not shown) on the K-wire 46and/or dilator 48 (or a control element coupled thereto), indicating theorientation of the electrode 70 to the user.

In the embodiment shown, the trajectory of the K-wire 46 and initialdilator 48 is such that they progress towards an intervertebral targetsite in a postero-lateral, trans-psoas fashion so as to avoid the bonyposterior elements of the spinal column. Once the K-wire 46 is dockedagainst the annulus of the particular intervertebral disk, cannulae ofincreasing diameter may then be guided over the previously installedcannula 48 until a desired lumen diameter is installed, as shown in FIG.17. By way of example only, the dilating cannulae 26 may range indiameter from 6 mm to 30 mm, with length generally decreasing withincreasing diameter size. Depth indicia 72 may be optionally providedalong the length of each dilating cannula 48 to aid the user in gaugingthe depth between the skin of the patient and the surgical target site.As shown in FIG. 18, the working cannula 50 may be slideably advancedover the last dilating cannula 48 after a desired level of tissuedilation has been achieved. As shown in FIG. 19, the last dilatingcannula 48 and then all the dilating cannulae 26 may then be removedfrom inside the inner lumen of the working cannula 50 to establish theoperative corridor therethrough.

During the advancement of the K-wire 46, each dilating cannula 48, andthe working cannula 50, the surgical system 20 will perform (under thedirection of a user) the nerve proximity and optionally nerve directionassessments according to the present invention. By way of example, thismay be explained with reference to FIGS. 20 and 21, which illustrateexemplary graphic user interface (GUI) screens provided on the screendisplay 40 for the purpose of allowing the user to control the surgicalsystem 20 to access a surgical target site according to the presentinvention. In one embodiment, the surgical system 20 initially operatesin a “DETECTION” mode, as shown in FIG. 20, wherein a mode label 80 willpreferably show the word “DETECTION” highlighted to denote the nerveproximity function of the present invention. A spine image 81 willpreferably be provided showing electrode placement on the body, withlabeled EMG channel number tabs 82 on each side (1-4 on left and right)capable of being highlighted or colored depending on the specificfunction being performed. A myotome label 83 is provided indicating themyotome associated with each EMG channel tab 81, including (optionally)the corresponding spinal level(s) associated with the channel ofinterest. A surgical accessory label 84 is provided indicating theparticular surgical accessory 30 being employed at any given time (i.e.“Dilating Cannula” to denote use of the sequential dilation accesssystem 34), as well as a “Dilator in Use” display 85 showing(graphically and numerically) the particular diameter of the dilatingcannula 48 in use. A threshold label 86 is also provided indicating thestimulation threshold required to elicit a measurable EMG response for agiven myotome. In one embodiment, this is situated, by way of exampleonly, within a cannula graphic 87 denoting a cross-section of thedilating cannula in use). A horizontal bar-chart 88 may also be providedindicating the stimulation level being emitted from the particularsurgical accessory in use.

Any number of the above-identified indicia (such as the threshold label86 and EMG channel tabs 82) may be color-coded to indicate generalproximity ranges (i.e. “green” for a range of stimulation thresholdsabove a predetermined safe value, “red” for range of stimulationthresholds below a predetermined unsafe value, and “yellow” for therange of stimulation thresholds in between the predetermined safe andunsafe values—designating caution). In one embodiment, “green” denotes astimulation threshold range of 9 milliamps (mA) or greater, “yellow”denotes a stimulation threshold range of 6-8 mA, and “red” denotes astimulation threshold range of 6 mA or below. An “Advance-or-Hold”display 89 may also be provided to aid the user in progressing safelythrough the tissue required to create the operative corridor. ADVANCEmay be highlighted indicating it is safe to advance the cannula (such aswhere the stimulation threshold is within the safe or “green” range).HOLD may be highlighted indicating to the user that the particularsurgical accessory may be too close to a nerve (such as where thestimulation threshold is within the “yellow” or “red” ranges) and/orthat the surgical system 20 is in the process of determining proximityand/or direction. In one embodiment, ADVANCE may be omitted, leaving itto the discretion of the user to advance the dilating cannula as soon asthe HOLD is no longer illuminated or highlighted.

Insertion and advancement of the access instruments 46-50 should beperformed at a rate sufficiently slow to allow the surgical system 20 toprovide real-time indication of the presence of nerves that may lie inthe path of the tip. To facilitate this, the threshold currentI_(Thresh) may be displayed such that it will indicate when thecomputation is finished and the data is accurate. For example, when theDETECTION information is up to date and the instrument such that it isnow ready to be advanced by the surgeon, it is contemplated to have thecolor display show up as saturated to communicate this fact to thesurgeon. During advancement of the instrument, if an EMG channel's colorrange changes from green to yellow, advancement should proceed moreslowly, with careful observation of the detection level. If the channelcolor stays yellow or turns green after further advancement, it is apossible indication that the instrument tip has passed, and is movingfarther away from the nerve. If after further advancement, however, thechannel color turns red, then it is a possible indication that theinstrument tip has moved closer to a nerve. At this point the displaywill show the value of the stimulation current threshold in mA. Furtheradvancement should be attempted only with extreme caution, whileobserving the threshold values, and only if the clinician deems it safe.If the clinician decides to advance the instrument tip further, anincrease in threshold value (e.g. from 3 mA to 4 mA) may indicate theInstrument tip has safely passed the nerve. It may also be an indicationthat the instrument tip has encountered and is compressing the nerve.The latter may be detected by listening for sporadic outbursts, or“pops”, of nerve activity on a free running EMG audio output formingpart of the surgical system 20.

Once a nerve is detected using the K-wire 46, dilating cannula 48, orthe working cannula 50, the surgeon may select the DIRECTION function todetermine the angular direction to the nerve relative to a referencemark on the access components 46-50, as shown in FIG. 21. In oneembodiment, a directional arrow 90 is provided, by way of example only,disposed around the cannula graphic 87 for the purpose of graphicallyindicating to the user what direction the nerve is relative to theaccess components 46-50. This information helps the surgeon avoid thenerve as he or she advances the cannula. In one embodiment, thisdirectional capability is accomplished by equipping the dilators 48 andworking cannula 50 with four (4) stimulation electrodes disposedorthogonally on their distal tip. These electrodes are preferablyscanned in a monopolar configuration (that is, using each of the 4electrodes as the stimulation source). The threshold current(I_(Thresh)) is found for each of the electrodes by measuring the muscleevoked potential response V_(pp) and comparing it to a known thresholdV_(thresh) From this information, the direction from a stimulationelectrode to a nerve may be determined according to the algorithm andtechnique set forth below and with immediate reference to FIG. 22. Thefour (4) electrodes are placed on the x and y axes of a two dimensionalcoordinate system at radius R from the origin. A vector is drawn fromthe origin along the axis corresponding to each electrode that has alength equal to I_(Thresh) for that electrode. The vector from theorigin to a direction pointing toward the nerve is then computed. Usingthe geometry shown, the (x,y) coordinates of the nerve, taken as asingle point, can be determined as a function of the distance from thenerve to each of four electrodes. This can be expressly mathematicallyas follows:

Where the “circles” denote the position of the electrode respective tothe origin or center of the cannula and the “octagon” denotes theposition of a nerve, and d₁, d₂, d₃, and d₄ denote the distance betweenthe nerve and electrodes 1-4 respectively, it can be shown that:

$x = {{\frac{d_{1}^{2} - d_{3}^{2}}{{- 4}R}\mspace{14mu}{and}\mspace{14mu} y} = \frac{d_{2}^{2} - d_{4}^{2}}{{- 4}R}}$

-   -   Where R is the cannula radius, standardized to 1, since angles        and not absolute values are measured.

After conversion from (x,y) to polar coordinates (r,θ), then θ is theangular direction to the nerve. This angular direction may then bedisplayed to the user, by way of example only, as the arrow 91 shown inFIG. 21 pointing towards the nerve. In this fashion, the surgeon canactively avoid the nerve, thereby increasing patient safety whileaccessing the surgical target site. The surgeon may select any one ofthe 4 channels available to perform the Direction Function. The surgeonshould preferably not move or rotate the instrument while using theDirection Function, but rather should return to the Detection Functionto continue advancing the instrument.

After establishing an operative corridor to a surgical target site viathe surgical access system 34 of the present invention, any number ofsuitable instruments and/or implants may be introduced into the surgicaltarget site depending upon the particular type of surgery and surgicalneed. By way of example only, in spinal applications, any number ofimplants and/or instruments may be introduced through the workingcannula 50, including but not limited to spinal fusion constructs (suchas allograft implants, ceramic implants, cages, mesh, etc . . . ),fixation devices (such as pedicle and/or facet screws and relatedtension bands or rod systems), and any number of motion-preservingdevices (including but not limited to total disc replacement systems).

II. Pedicle Integrity Assessment

With reference again to FIGS. 2-3, the surgical system 20 can also beemployed to perform pedicle integrity assessments via the use of pedicletesting assembly 36. More specifically, The pedicle testing assembly 36of the present invention is used to test the integrity of pedicle holes(after formation) and/or screws (after introduction). The pedicletesting assembly 36 includes a handle assembly 54 and a probe member 56having a generally ball-tipped end 60. The handle 54 may be equippedwith a mechanism (via hardware and/or software) to identify itself tothe surgical system 20 when it is attached. In one embodiment, the probemember 56 is disposable and the handle 54 is reusable and sterilizable.The handle 54 may be equipped with one or more buttons for selectivelyapplying the electrical stimulation to the ball-tipped end 60 at the endof the probe member 56. In use, the ball tip 60 of the probe member 56is placed in the screw hole prior to screw insertion or placed on theinstalled screw head and then stimulated to initiate the pedicleintegrity assessment function of the present invention. As will beexplained in greater detail below, it may also applied directly to anerve to obtain a baseline current threshold level before testing eitherthe screw hole or screw. If the pedicle wall has been breached by thescrew or tap or other device employed to form the screw hole, thestimulation current will pass through the bone to the adjacent nerveroots such that they will depolarize at a lower stimulation current.

Upon pressing the button on the screw test handle 54, the software willexecute a testing algorithm to apply a stimulation current to theparticular target (i.e. screw hole, inserted pedicle screw, or barenerve), setting in motion the pedicle integrity assessment function ofthe present invention. The pedicle integrity assessment features of thepresent invention may include, by way of example only, an “Actual” mode(FIGS. 23-24) for displaying the actual stimulation threshold 91measured for a given myotome, as well as a “Relative” mode (FIGS. 25-27)for displaying the difference 92 between a baseline stimulationthreshold assessment 93 of a bare nerve root and an actual stimulationthreshold assessment 91 for a given myotome. In either case, thesurgical accessory label 84 displays the word “SCREW TEST” to denote useof the pedicle testing assembly 36 for performing pedicle integrityassessments. The screw test algorithm according to the present inventionpreferably determines the depolarization (threshold) current for allresponding EMG channels. In one embodiment, the EMG channel tabs 82 maybe configured such that the EMG channel having the lowest stimulationthreshold will be automatically enlarged and/or highlighted and/orcolored (EMG channel tab R3 as shown in FIG. 23) to clearly indicatethis fact to the user. As shown in FIG. 24, this feature may beoverridden by manually selecting another EMG channel tab (such as EMGchannel tab R1 in FIG. 24) by touching the particular EMG channel tab 82on the touch screen display 40. In this instance, a warning symbol 94may be provided next to the EMG channel tab having the loweststimulation threshold (once again, EMG channel tab R3 in FIG. 23) toinform the user that the stimulation threshold 91 is not the loweststimulation threshold.

Any number of the above-identified indicia (such as the baselinestimulation 93, actual stimulation 91, difference 92, and EMG channeltabs 82) may be color-coded to indicate general safety ranges (i.e.“green” for a range of stimulation thresholds above a predetermined safevalue, “red” for range of stimulation thresholds below a predeterminedunsafe value, and “yellow” for the range of stimulation thresholds inbetween the predetermined safe and unsafe values—designating caution).In one embodiment, “green” denotes a stimulation threshold range of 9milliamps (mA) or greater, “yellow” denotes a stimulation thresholdrange of 6-8 mA, and “red” denotes a stimulation threshold range of 6 mAor below. By providing this information graphically, a surgeon mayquickly and easily test to determine if the integrity of a pedicle hasbeen breached or otherwise compromised, such as may result due to theformation of a pedicle screw hole and/or introduction of a pediclescrew. More specifically, if after stimulating the screw hole and/orpedicle screw itself the stimulation threshold is: (a) at or below 6 mA,the threshold display 40 will illuminate “red” and thus indicate to thesurgeon that a breach is likely; (b) between 6 and 8 mA, the thresholddisplay 40 will illuminate “yellow” and thus indicate to the surgeonthat a breach is possible; and/or (c) at or above 8 mA, the thresholddisplay 40 will illuminate “green” and thus indicate to the surgeon thata breach is unlikely. If a breach is possible or likely (that is,“yellow” or “red”), the surgeon may choose to withdraw the pedicle screwand redirect it along a different trajectory to ensure the pedicle screwno longer breaches (or comes close to breaching) the medial wall of thepedicle.

III. Neural Pathology Monitoring

The surgical system 20 may also be employed to perform neural pathologymonitoring. As used herein, “neural pathology monitoring” is defined toinclude monitoring the effect of nerve retraction over time (“nerveretraction monitoring”), as well as monitoring the effect of a surgeryon a particular unhealthy nerve (“surgical effect monitoring”). Theformer—nerve retraction monitoring—is advantageous in that it informsthe surgeon if, and the extent to which, such retraction is degrading ordamaging an otherwise healthy nerve under retraction. Thelatter—surgical effect monitoring—is advantageous in that it informs thesurgeon if, and the extent to which, the given surgical procedure isimproving or aiding a previously unhealthy nerve. In both cases, thequalitative assessment of improvement or degradation of nerve functionmay be defined, by way of example, based on one or more of thestimulation threshold (I_(Thresh)), the slope of the EMG response (uV)versus the corresponding stimulation threshold (I_(Thresh)), and/or thesaturation or maximum EMG response (V_(pp)) for a given nerve root beingmonitored.

FIG. 28 illustrates this important aspect of the present invention,noting the differences between a healthy nerve (A) and an unhealthynerve (B). The inventors have found through experimentation thatinformation regarding nerve pathology (or “health” or “status”) can beextracted from recruitment curves generated according to the presentinvention. In particular, it has been found that a healthy nerve ornerve bundle will produce a recruitment curve having a generally lowcurrent threshold (I_(Thresh)), a linear region having a relativelysteep slope, and a relatively high saturation region (similar to thoseshown on recruitment curve “A” in FIG. 28). On the contrary, a nerve ornerve bundle that is unhealthy or whose function is otherwisecompromised or impaired (such as being impinged by spinal structures orby prolonged retraction) will produce recruitment curve having agenerally higher threshold, a linear region of reduced slope, and arelatively low saturation region (similar to those shown on recruitmentcurve “B” in FIG. 28). By recognizing these characteristics, one canmonitor a nerve root being retracted during a procedure to determine ifits pathology or health is affected (i.e. negatively) by suchretraction. Moreover, one can monitor a nerve root that has already beendeemed pathologic or unhealthy before the procedure (such as may becaused by being impinged by bony structures or a bulging annulus) todetermine if its pathology or health is affected (i.e. positively) bythe procedure.

The nerve root retractor assembly 38 shown in FIG. 2 is capable ofperforming both types of neural pathology monitoring. However, based onits particular shape and configuration (being bent and suitably shapedto hook and thereafter move a nerve root out of a surgical target site),it is better suited to perform “nerve retraction monitoring.” Withcombined reference to FIGS. 2 and 29-31, the nerve root retractorassembly 38 includes the same style surgical accessory handle assembly54 as employed with in the pedicle testing assembly 36. The nerve rootretractor 62 has a generally angled orientation relative to thelongitudinal axis of the handle assembly 54. The distal end 64 isgenerally curved and includes an arcuate nerve engagement surface 66equipped with, by way of example only, two stimulation electrodes 100.As best shown in FIG. 31, the nerve root retractor 62 is preferablyremovable from the handle assembly 36. To accomplish this, the handleassembly 54 includes a detachable cap member 102. Threads 104 areprovided on the proximal end of the nerve root retractor 62 to allow athreaded coupling engagement between the handle assembly 54 and thenerve root retractor 62. During such engagement, electrical contacts 106on the nerve root retractor 62 becomes electrically coupled to thehandle assembly 54 such that, upon activation of one or more of thebuttons 108, 110, a stimulation current signal will be transmitted fromthe control unit 22 and/or patient module 24 and delivered to thestimulation electrodes 100 on the nerve root retractor 62 for thepurpose of performing neural pathology monitoring according to thepresent invention. The nerve root retractor 62 is preferably disposableand, as described above, the handle assembly 54 is reusable andsterilizable.

In use, the nerve root retractor 62 is introduced into or near asurgical target site in order to hook and retract a given nerve out ofthe way. According to the present invention, the nerve root may bestimulated (monopolar or bipolar) before, during, and/or afterretraction in order to assess the degree to which such retractionimpairs or otherwise degrades nerve function over time. To do so, theuser may operate one or more buttons 108, 110 of the handle assembly 54to selectively transmit a stimulation signal (preferably, a currentsignal) from the patient module 24 to the electrode(s) on the engagementsurface 66 of the nerve root retractor 62. By monitoring the myotomeassociated with the nerve root being retracted (via the EMG harness 26)and assessing the resulting EMG responses (via the control unit 22), thesurgical system 20 can assess whether (and the degree to which) suchretraction impairs or adversely affects nerve function over time. Withthis information, a user may wish to periodically release the nerve rootfrom retraction to allow nerve function to recover, thereby preventingor minimizing the risk of long-term or irreversible nerve impairment. Aswill be described in greater detail below, a similar neural pathologyassessment can be undertaken, whereby an unhealthy nerve may bemonitored to determine if nerve function improves due to a particularsurgical procedure, such as spinal nerve decompression surgery.

The nerve retraction monitoring feature of the present invention is bestviewed with regard to FIGS. 32 and 33. The neural pathology screendisplay 40 may include any of a variety of indicia capable ofcommunicating parameters associated with the nerve retraction monitoringfeature of the present invention to a surgeon, including but not limitedto (in FIG. 32) a pre-operative recruitment curve graph 120, anintra-operative recruitment curve graph 122, and a differential display124 indicating the relative difference between the stimulationthreshold, slope, and saturation before the surgery and during thesurgery. In this manner, the surgeon may intra-operatively assess if theretracted nerve is being damaged or otherwise compromised (such as dueto a prolonged surgery), such that it can be temporarily released toallow it to recover before returning to retraction to continue with thesurgery. It's believed that releasing the nerve root in this fashionwill prevent or reduce the adverse effects (nerve function compromise)that may otherwise result from prolonged retraction.

FIG. 33 shows an alternate screen display including a stimulationthreshold vs. time graph 130, slope vs. time graph 132, and saturationvs. time graph 134 for a given healthy nerve (as measured at aparticular myotome) during nerve retraction monitoring. As will beappreciated, the start of nerve retraction initiates a progressiveincrease in stimulation threshold 130 and a concomitant progressivedecrease in slope 132 and saturation 134, all of which cease and reverseat or close to the point the retraction is stopped. By monitoring thisinformation, a surgeon can effectively determine when the nerve is inneed of being released and, after that point, when it is generally safeto resume retraction.

The surgical effect nerve monitoring of the present invention is bestviewed with regard to FIGS. 34 and 35. The neural pathology screendisplay 40 may include any of a variety of indicia capable ofcommunicating parameters associated with the surgical effect nervemonitoring feature of the present invention to a surgeon, including butnot limited to (in FIG. 34) a pre-operative recruitment curve graph 140,a post-operative recruitment curve graph 142, and a differential display144 indicating the relative difference between the stimulationthreshold, slope, and saturation before the surgery and after thesurgery. In this manner, the surgeon may determine whether a previouslyunhealthy nerve has been positively affected by the surgery. This isparticularly advantageous in assessing the effectiveness of spinaldecompression surgery, wherein the effectiveness of the decompressionmay be determined by identifying whether the health of the compressednerve root improves as a result of the surgery. This determination mayalso be made, by way of example, by (see FIG. 35) displaying variousgraphs to the user, such as a stimulation threshold vs. time graph 150,a slope vs. time graph 152, and saturation vs. time graph 154 for agiven unhealthy nerve (as measured at a particular myotome) before,during, and after surgery. As can be seen, an improvement in nervefunction due to surgery will cause the stimulation threshold to decreasepost-operatively and the slope and saturation to increasepost-operatively.

Although not shown, it is to be readily appreciated that the nerveretraction monitoring and surgical effect nerve monitoring techniquesdescribed above (both of which form part of the neural pathologymonitoring feature of the present invention), should preferably beperformed on different myotomes in that the former technique isparticularly suited for assessing a healthy nerve and the latter isparticularly suited for assessing an unhealthy nerve. Moreover, althoughnot shown in FIGS. 32-35, the various graphs may be formed based on acompilation of EMG responses from more than one myotome withoutdeparting from the scope of the present invention.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the invention will typicallybe stored in one or more machine readable storage mediums such as fixed(hard) drives, diskettes, optical disks, magnetic tape, semiconductormemories such as ROMs, PROMs, etc., thereby making an article ofmanufacture in accordance with the invention. The article of manufacturecontaining the computer programming code is used by either executing thecode directly from the storage device, by copying the code from thestorage device into another storage device such as a hard disk, RAM,etc. or by transmitting the code on a network for remote execution. Ascan be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the scope of the appended claims.

1. A system for forming a path to a spinal target site via a trans-psoasapproach, comprising: a dilating cannula having longitudinal axis, adistal end, a proximal end, and a length such that said proximal endextends beyond a skin surface when said distal end is positionedadjacent to said spinal target site via a trans-psoas approach to saidspinal target site, said dilator being insulated along the entire lengthwith the exception of at least one exposed electrical contact at saidproximal end and an exposed stimulation electrode at said distal end,said stimulation electrode being exposed along only a radial portion ofsaid distal end, said dilating cannula further including a referencemark viewable when said distal end of said dilating cannula is locatedbetween said skin surface and said spinal target site and indicative ofthe radial position of said exposed stimulation electrode.
 2. The systemof claim 1, wherein said stimulation electrode is positioned at an anglerelative to said longitudinal axis.
 3. The system of claim 2, whereinsaid angle is between 5 and 85 degrees.
 4. The system of claim 1,further including a stimulation connector with a first end releasablyattachable to said proximal end of said dilating cannula and a cable forestablishing electrical communication between said stimulation connectorand a nerve monitoring system, said first end establishing electricalcommunication between said at least one electrical contact of saiddilating cannula and said connector when said stimulation connector isattached to said dilating cannula.
 5. The system of claim 4, whereinsaid stimulation connector is a clamp.
 6. The system of claim 1, furthercomprising a second dilating cannula that is slidable over the firstdilating cannula.
 7. The system of claim 6, wherein said second dilatingcannula includes a longitudinal axis, a distal end, a proximal end, anda length, said second dilator being insulated along the entire lengthwith the exception of at least one exposed electrical contact at saidproximal end and an exposed stimulation electrode at said distal end,said stimulation electrode being exposed along only a radial portion ofdistal end, said second dilating cannula further including a referencemark viewable when said distal end of said second dilating cannula islocated between said skin surface and said spinal target site andindicative of the radial position of said exposed stimulation electrode.8. The system of claim 7, further including a stimulation connectorhaving a first end attachable to said proximal end of each of said firstdilating cannula and said second dilating cannula and a cable forestablishing electrical communication between said stimulation connectorand an electrical stimulator, said first end establishing electricalcommunication between said at least one electrical contact of said firstdilating cannula or said second dialting cannula and said stimulationconnector when said stimulation connector is attached to said firstdilating cannula or said second dilating cannula.
 9. The system of claim8, further comprising a working corridor instrument that is slidableover the second dilating cannula to form a trans-psoas operativecorridor to the spinal target site along said path created by said firstdilating cannula.
 10. The system of claim 4, further comprising acontrol unit operable to command delivery of an electrical stimulationsignal from a stimulator in communication with said stimulationconnector.
 11. The system of claim 10, wherein said control unit isoperable to receive user input regarding delivery of said stimulationsignal and to display on a color display device neuromuscular responsedata detected in response to the stimulation signal.
 12. The surgicalsystem of claim 11, wherein the color display device of the control unitcomprises a touchscreen device to receive the user input regardingdelivery of a stimulation signal.
 13. The surgical system of claim 11,wherein the neuromuscular response data displayed comprises a numericstimulation threshold current level that indicates an amplitude of thestimulation current pulses required to evoke a neuromuscular responsehaving an amplitude value greater than a predetermined value.
 14. Thesurgical system of claim 13, wherein the control unit display alsodisplays a color associated with the numeric stimulation threshold. 15.A surgical system for forming a path to a spinal target site via atrans-psoas approach, comprising: a sequential dilation access systemcomprising a plurality of dilating cannulas to form a trans-psoascorridor between a skin surface and a targeted spine site, the pluralityof dilating cannulas comprising an outer dilating cannula fitting overanother of the dilating cannulas when advanced in a trans-psoas pathtoward the targeted spine site, wherein a stimulation electrode ispositioned on at least one of the dilating cannulas to deliver astimulation signal for nerve monitoring proximate to a distal end of thedilating cannula when advanced in the trans-psoas path, the stimulationelectrode being arranged in a fixed position relative to a longitudinalaxis of the at least one dilating cannula such that the stimulationelectrode rotates with the at least one dilating cannula when the atleast one dilating cannula is rotated about the longitudinal axis. 16.The system of claim 15, wherein said stimulation electrode is positionedat an angle relative to said longitudinal axis of said at least onedilating cannula.
 17. The system of claim 16, wherein said angle isbetween 5 and 85 degrees.
 18. The system of claim 15, wherein said atleast one dilating cannula includes a reference mark proximate aproximal end of the at least one dilating cannula that is indicative ofthe radial position of the stimulation electrode on the at least onedilating cannula.
 19. The system of claim 15, wherein the plurality ofdilating cannulas of the sequential dilation access system range indiameter from 6 mm to 30 mm.
 20. The system of claim 15, wherein thelength of each the plurality of dilating cannulas of the sequentialdilation access system decreases with increasing diameter.
 21. Thesystem of claim 15, wherein at least one of the plurality of dilatingcannulas includes depth markings to aid in gauging the depth to thesurgical target site.
 22. The system of claim 15, further including aworking corridor instrument that is slidable over the outer dilatingcannula, the working corridor instrument being configured to maintain atrans-psoas operative corridor to the targeted spine site after theplurality of dilating cannulas are removed from the working corridor.23. The system of claim 22, wherein the working corridor instrumentcomprises a working cannula structure.